Hola! Soy Elver Sena Sosa. And now we're gonna talk about vSAN failures.
but in particular we're going to talk about three types of failures
which if you deal with storage you know that you have permanent device loss or
path down and now because we're using the network we're gonna have middle
partition. So, how does vSAN, vSphere handle these types of failures? So, we're
going to assume we have a VM with a B indicate object that has the FTT policy
of 1 and a RAID of 1. What I will be describing it will be similar to the
other types of entity processes options that you have for the SPBM
so just to keep simple use one with a rate one it's easy to see. So, the first
one we want to talk about is a permanent device loss. Permanent device loss means
that I am a hundred percent certain that you are dead that's what it means
and usually that means that whoever died or it's about to die sends a message
that say "Hey I'm dying save yourselves" Right? So, the host that has the VM decay,
VM that is running it, is sending commands to hear and hear or
reads. Right? High-opps. You're sending it to both of them now what happens if let's
say component number one either the capacity disk itself assessing error
message it's more alarm saying "Hey I died" or the SSD the cache drive says "Hey I'm dead"
In both of those cases the component is unavailable. In the case of
the cache being the one that dies that means that the whole disk group is
unavailable which means that every component of this group is unavailable.
In either case, the host will tell the vSAN cluster
"Hey dude that component yeah it's not available anymore sorry". So that happens right away.
no delay basically. So when that happens the source host immediately works with
the vSAN cluster and goes "Hey I need another place to put that component
because I have an SPBM of FTT of 1 and a RAID of 1 so I'm not in compliance right now"
So when that happens for that brief for that moment of time but you
don't have the second component the object itself is something as the
degraded state. So anytime you hear "degraded state" in this sense
that means that you having a PDL situation and the cluster at this moment
is working to recover that loss component. In this scenario we have a
host this one, and that one, one both of them, that could be candidates for the
component 1. So let's assume that this host is selected to take component 1
in that case what happens immediately as this as soon as this guy's selected
component 2, the other replica, this host starts replicating itself over so this
component can match this one. At the same time the source host right away it
starts sending over the new rights to this guy, new rights to this guy, new rights
to this guy, and that helps us speed up the recovery. So that's what happens with
a permit device loss. We know somebody's dead, we need to recover right away. If by
any for whatever reasons there is not even a physical component said you only
had a three host cluster, your SPBM policy will stay as degraded for that
VM because there's no other component that can be leveraged to satisfy the
policy. Now in this example I put a single disk group in each host. If in
the case that the capacity disk dies here, the other capacity disk will
be candidate to host that component. If the disk group dies and this host has
no other disk groups, then yeah you have to go to another host. Right so that's
permanent device loss. What does all path down means? It means that I can't
I don't hear from you! Where are you? Where did you go? Something happened to you but
I'm not sure if you're dead, you went on vacation, you're having lunch. I have no idea
what happened to you. So, when there is an all path down, vSAN by default goes:
"Okay I haven't heard from this guy, let's say the host, somebody unplug the
network cable or somebody pull out the power cord which never happens. So this
host is no longer responding. So by default ESKi host is gonna go
"Okay I'm gonna wait about 5 seconds after those 5 seconds I'm going to
assume that you are not coming back anytime soon so I'm to continue to
write over here right right over here" and I'm gonna wait 60 minutes
by default before I changed your policy from absent
right and I start recovering and when I start recovering I will follow the same
process I followed before I will find another candidate to put the component
that's missing over there I will start writing over here and this guy will
start replicating. However, if doing that process you come back I'm gonna, vSAN goes:
"Wait a minute you came back so what's different between the time you
were gone and now what's the difference between this and how much do I have to
go to finish this replication". And vSAN will determine is it more efficient
to continue replicating this way? or go back to the other guy and just give him
what he's missing? Another thing about all path down that
I mentioned about host being unavailable but in all paths down can also be that
somebody walk into your server room at the data center and pull out the disk.
There was no error message so as far as we know we don't know what happened to
the disk. So that is also another path down. And finally the other failure which
is network partition will be similar to doing this. These two guys cannot see
these three guys now from a vSAN perspective, these two, what's happening
here is that remember the components are over here just to keep it simple
this VM is no longer reaching the vSAN data store. So decent
vSAN, vSphere HA comes in kicks in and goes " Oh that host is isolated you should
have a PDL, APD or shut down the VM" when this happens so vSphere HA will shut
down the VM and HA will recover that that VM over here. As for all the hosts
are concerned if they were holding any other components of a VM because they're
no longer in the majority here the witness is here and the components is here
let's say, then that guy would tell the HA "Hey I'm the only guy from that
component I'm one of three therefore are no longer available you gotta use those
over there". So that kind of covers it for how vSAN handles failures.
Thank you for watching! Yo soy Elver Sena Sosa.